With vehicles driven by means of the front axle or the rear axle, the front axle rotates (average speed of the two front wheels) slightly differently to the rear axle (average speed of the two rear wheels). The revolution rate difference here is the result of different dynamic tire radii, manufacturing tolerances, different tire wear, different axle loads, differences in the tire pressure, mixed tires and wheel slip caused by the drive torque. During cornering the revolution rate difference is the result of the different rolling distances travelled by the wheels.
With all-wheel drive vehicles—for example a vehicle with front wheel drive and a rear wheel drive that can be selected by means of a clutch—a further revolution rate difference can be added because of a difference in transmission ratios in the axle gears.
When the driving situation of straight line travel at constant speed is considered, a revolution rate difference of <1% is usually detected. In said driving situation the all-wheel drive is not required, but which of the two axles is the faster and which is the slower depends in particular on the dynamic radii of the tires and is random. With the all-wheel drive clutch engaged or the centre differential locked, losses therefore result, the speed difference decreasing by means of the tire slip.
When the drive torques to be transferred by the drive shafts and propeller shaft during an acceleration process are considered, it often appears that the torque of the front axle undergoes a change of sign when transitioning to a constant speed, i.e. when the acceleration process finishes. The front axle changes from traction mode to drag mode. The front axle brakes, the rear axle pushes a little more and the drive system runs under stress. A reactive torque flow results, which overall causes higher torques in the front axle and in the rear axle. With a gearbox the efficiency in the traction mode is generally greater than in the drag mode, so that the indicated losses are even greater.
A method for controlling a controllable clutch in the drive train between the front axle and the rear axle of a four-wheel drive is described in DE 197 006 720 A1. A wheel revolution rate sensor is associated with each of the wheels—the signals of said sensors are analyzed. A theoretical revolution rate difference of the clutch is determined for the drive mode without wheel slip, the difference being determined from the speed of travel, the radius of the turn and the different wheel radii. A revolution rate difference for which turning occurs without wheel slip is determined, and the clutch is adjusted by means of a generated control signal according to said revolution rate difference value.
DE 36 26 025 A1 presents is similarly operating drive device for an all-wheel drive vehicle with a friction plate clutch for variable transfer of a drive torque to the front wheels.
With the four-wheel drive according to DE 37 21 626 C2, to improve the braking behavior the clutch in the drive train between the front axle and the rear axle is disengaged on exceeding a specified revolution rate reduction gradient.
DE 36 21 225 C1 discloses an all-wheel drive with a permanently acting rear wheel drive and a front axle drive that can be selected by means of an electrohydraulically controlled clutch. On exceeding a slip threshold, the clutch is briefly disengaged, which prevents stresses in the drive train.
Controlling a clutch in the drive train between the front and rear axles of a vehicle with four-wheel drive according to DE 102 60 196 A1 has the effect that the clutch is always subjected to torque, i.e. is never fully disengaged. A better transition in the event of requiring all-wheel drive should thus be achieved.
With DE 69 025 487 T2, for drive force distribution a correction factor is determined for determining the true speed on the basis of the revolution rate difference and the measured speed.